Engine components in the hot gas flow of modern combustion turbines are required to operate at ever-increasing temperatures as engine efficiency requirements continue to advance. Ceramics typically have higher heat tolerance and lower thermal conductivities than metals, particularly in the case of oxide-based ceramic materials. For this reason, ceramics have been used both as structural materials in place of metallic materials and as coatings for both metal and ceramic structures. Ceramic matrix composite (CMC) wall structures with ceramic insulation outer coatings, such as described in commonly owned U.S. Pat. No. 6,197,424, have been developed to provide components with the high temperature stability of ceramics without the brittleness of monolithic ceramics.
Film cooling is sometimes used to reduce the temperature of the hot working gas along the surface of components, thereby lowering the heat load on the component. This requires a large volume of cooling air to be supplied through many film channels and outlets across the width and length of the component surface. Convective or impingement cooling on back surfaces of component walls is also used to remove heat passing through the walls. However, backside cooling efficiency is reduced by the low thermal conductivity of ceramic material and by the fact that the wall thickness of a CMC structure may be thicker than in an equivalent metal structure.
Commonly owned U.S. Pat. No. 6,709,230 describes cooling channels in a ceramic core of a gas turbine vane behind an outer CMC airfoil shell. Commonly owned U.S. Pat. No. 6,746,755 uses ceramic matrix composite cooling tubes between CMC face sheets to form a CMC wall structure with internal cooling channels. Commonly owned U.S. Pat. No. 6,984,277 describes surface undulations on a CMC surface for bond enhancement. Further improvements in the cooling of a ceramic matrix composite wall structure are desired to support further increases in the firing temperatures of advanced gas turbine engines.